1. Field of the Invention
The present invention relates generally to optical fiber coating compositions, and, more particularly, to fiber optic coating compositions that include a secondary or tertiary amino silicone-containing release agent, and to optical fibers coated with such compositions.
2. Description of Related Art
Optical glass fibers are frequently coated with two or more superposed radiation-curable coatings which together form a primary coating immediately after the glass fiber is produced by drawing in a furnace. The coating which directly contacts the optical glass fiber is called the “inner primary coating” and an overlaying coating is called the “outer primary coating.” In older references, the inner primary coating was often called simply the “primary coating” and the outer primary coating was called a “secondary coating,” but for reasons of clarity, that terminology has been abandoned by the industry in recent years. Inner primary coatings are softer than outer primary coatings.
Single-layered coatings (“single coatings”) can also be used to coat optical fibers. Single coatings generally have properties (e.g., hardness) which are intermediate to the properties of the softer inner primary and harder outer primary coatings.
The relatively soft inner primary coating provides resistance to microbending which results in attenuation of the signal transmission capability of the coated optical fiber and is, therefore, undesirable. The harder outer primary coating provides resistance to handling forces such as those encountered when the coated fiber is ribboned and/or cabled.
Optical fiber coating compositions, whether they are inner primary coatings, outer primary coatings, or single coatings, generally comprise, before cure, a polyethylenically-unsaturated monomer or oligomer dissolved or dispersed in a liquid ethylenically-unsaturated medium and a photoinitiator. The coating composition is typically applied to the optical fiber in liquid form and then exposed to actinic radiation to effect cure.
For the purpose of multi-channel transmission, optical fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical fiber assemblies include ribbon assemblies and cables. A typical ribbon assembly is made by bonding together a plurality of parallel oriented, individually coated optical fibers with a matrix material. The matrix material has the function of holding the individual optical fibers in alignment and protecting the fibers during handling and installation. Often, the fibers are arranged in “tape-like” ribbon structures, having a generally flat, strand-like structure containing generally from about 2 to 24 fibers. Depending upon the application, a plurality of ribbon assemblies can be combined into a cable which has from several up to about one thousand individually coated optical fibers. An example of a ribbon assembly is described in published European patent application No. 194891. A plurality of ribbon assemblies may be combined together in a cable, as disclosed, for example, in U.S. Pat. No. 4,906,067.
The term “ribbon assembly” includes not only the tape-like ribbon assembly described above, but optical fiber bundles as well. Optical fiber bundles can be, for example, a substantially circular array having at least one central fiber surrounded by a plurality of other optical fibers. Alternatively, the bundle may have other cross-sectional shapes such as square, trapezoid, and the like.
Coated optical fibers (or waveguides) whether glass, or, as has come into use more recently, plastic, for use in optical fiber assemblies are usually colored to facilitate identification of the individual coated optical fibers. Typically, optical fibers are coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to impart the desired color.
Typically, the matrix material of a fiber optic ribbon assembly or cable is separated from the individual coated fibers in order to facilitate splicing two cables, or the connection of a fiber to an input or output. It is highly desirable that the matrix material can be removed from the coated fiber with little or no effect on the outer primary coating or colored ink coating of the fiber. Good removability of the matrix material not only preserves the readily visual identification of the color coded fiber, it also avoids harming the waveguide during the removal process.
Heretofore, certain types of silicone-containing compounds have been suggested for inclusion in the coloring layer and/or matrix materials to enhance the removability of the matrix material of the ribbon from color coated optical fiber. For example, U.S. Pat. No. 4,828,349 describes a multi-core optical fiber unit in which each optical fiber element is coated with a peel layer and bound into a ribbon or cable with a covering layer. The peel layer of each optical fiber is comprised of an ultraviolate curing or thermosetting fluorocarbon resin or an ultraviolet curing or thermosetting silicone resin.
U.S. Pat. No. 5,621,838 discloses a multi-fiber coated optical fiber unit in which each of the coated optical fibers has a coloring layer as the outermost layer and is coated with a bundling or matrix layer. The coloring layers on the coated fibers and the matrix layer both contain a releasing agent consisting of a silicon resin or oil or a fluororesin or fluoro-oil as known in the art at the time the application for the patent was filed.
Japan laid-opened patent H1-152405 discloses an optical fiber taped core wire in which a plurality of optical core wires arranged in a plane are coated into a single unit. Each of the optical fiber core wires is coated with an ultraviolet cured resin and has a colored layer as its outermost layer. The colored layer includes an organic polysiloxane compound to improve the removability of the matrix coating. The organic polysiloxane compound includes at least one functional group, including an amino group.
Ink compositions which include silicone-based release agents have been used in the art, but are not entirely satisfactory. Silicone release agents, particularly those that are not reactive under the UV cure conditions, have been found to migrate in the ribbon assembly over time. Migration alters the release characteristics of the matrix materials from the outermost coating on the fiber optic, i.e., the outer primary coating or the ink coating, which is undesirable. Silicone release agents are also known to impair the stability of the pigment dispersion in the ink composition leading to pigment flocculation, particularly when the silicone release agent is added to the finished ink composition. Silicone release agents can also reduce the efficiency of manufacturing optical fiber ribbon by slowing line speed and reducing cure rate of the ink composition.
Despite the efforts of the prior art to provide optical fiber coating compositions and ink compositions in which, when a plurality of optical fibers are formed into a ribbon assembly, the matrix material of the ribbon assembly is easily removed from the ink composition, there remains a need for new release agents for ink compositions which allow for improved release of the matrix material, improve the stability of the pigment dispersion in the ink composition even when the release agent is added to the finished ink composition, and allow for improved efficiency and speed of manufacture of optical fiber ribbon.